This study examined the additive effects of social support and negative interactions in various relationship domains and the cross-domain buffering effects of social support on the detrimental impact of negative interactions on mental health among older adults in Japan. Data were obtained from a survey of residents of 30 municipalities in the Tokyo metropolitan area (N = 1,592). The results indicated that family members living together may share ambivalent social ties, anchored in positive sentiments and serving as sources of support but where criticism and excessive demands may occur. We found that negative interactions had a more potent additive effect on mental health. Moreover, the interaction effects of negative interactions with family and social support from other relatives suggested reverse buffering. Our findings suggest that interventions might be more necessary to cope with the negative social exchanges of close kin relationships among the elderly Japanese.